ci: add multi-arch CI and DESTDIR-staged autopkgtests for libppd

[rkt0209]

Summary

Adds a multi-architecture CI workflow and runs the downstream Debian
autopkgtests against an in-tree DESTDIR-staged build, so libppd is tested
end-to-end on every push/PR without needing the package installed system-wide.

What's included

• Multi-arch CI (.github/workflows/build.yml): builds libppd and runs
  make check on four architectures — amd64, arm64 (native) and armhf,
  riscv64 (QEMU-emulated).
• DESTDIR staging harness (Makefile.am, ci/autopkgtest/): installs
  into an ephemeral _ciroot tree and runs both downstream tests
  (libppd-2-dev, libppd-2-ppd-handling) against it via
  PATH / LD_LIBRARY_PATH / PKG_CONFIG_PATH pointed at the staged tree.
• No absolute paths / no privilege: the libppd-2-ppd-handling test
  reads LIBPPD_TESTPPD / LIBPPD_PPDDIR env overrides that default to the
  original /usr paths (fully backward-compatible with the real Debian
  autopkgtest). This replaces the earlier bind-mount/proot redirection, so the
  full suite runs unprivileged and identically on native and emulated legs.

Notes

• libppd.pc.in: Cflags now exports -I${includedir} -I${includedir}/ppd
  (was -I${includedir}/ppd only). Headers install under $(includedir)/ppd/
  and consumers use <ppd/ppd.h>, which needs -I${includedir}. The old flag
  is kept for backward compatibility — this is a genuine .pc fix, not just a
  CI workaround.
• Build requires ./configure --enable-ppdc-utils (the libppd-2-dev test
  compiles a .drv with the staged ppdc).